Compression ignition engines provide advantages in fuel economy, but produce both nitrogen oxides (NOx) and particulates during normal operation. New and existing regulations continually challenge manufacturers to achieve good fuel economy and reduce the particulates and NOx in the exhaust gas of the engine. Lean-burn engines may achieve the fuel economy objective, but the high concentrations of oxygen in the exhaust of these engines yield relatively high concentrations of NOx. Accordingly, vehicles having compression engines may include after-treatment systems that attempt to reduce and/or eliminate NOx that is present in the exhaust gas.
One aspect of controlling such emissions by an after-treatment system includes injecting ammonia (NH3) into an exhaust gas that is generated by the combustion of a fossil fuel, such as, for example, diesel fuel, petroleum, and gasoline. The NH3 may be injected into the exhaust gas in dosing amounts or rates for converting NOx in the exhaust gas into at least nitrogen, such as, for example, through a catalytic reaction in an selective catalytic reduction (SCR) system. NH3 used by the after-treatment system in the conversion of NOx may be stored in, and supplied from, a cartridge.
The release and/or amount of NH3 delivered from the cartridge for injection into the exhaust gas may be controlled and/or monitored by an engine control unit or module (ECU). Such monitoring may be useful in estimating the amount of NH3 that has injected into the NOx for purposes of estimating the amount of NOx that has been converted into at least nitrogen. Additionally, such monitoring may provide information relating to the amount of NH3 that has been depleted from, or is remaining in, the cartridge. Typically, the quantity of NH3 injected into the exhaust gas is monitored, such as, for example, through the use of flow sensors that provide information to the ECU. However, flow sensors are relatively expensive, and require additional associated components or equipment, and incur installation expenses. Further, the ECU may require programming that allows the ECU to interpret and/or apply the information received from the flow sensor. Additionally, flow sensors may have durability and accuracy issues, particularly relating to the effects the harsh operating environment of an after-treatment system may have on the flow sensors.